A rear axle setup developed by Count de Dion in the 19th century in which the driving wheels are attached to curved dead axle that is attached to the frame by a central pivot, the differential unit is bolted to the frame and is connected to the driving wheels by drive axles using universal joints. The De Dion system keeps the wheels upright (the same as a live axle does), but unsprung weight is reduced because the differential is out of the axle. De Dion suspension also leaves room around the differential for inboard brakes, which can further reduce unsprung weight.
thanks to wings and wheels site for the cad jpeg
Now we know where it got its funny name
[Edited on 29-2-04 by mangogrooveworkshop]